There are numerous prior art techniques for measuring capacitance. Since it turns out that a capacitor is often better thought of as a machine or complex mechanism in its own right, rather than as an ideal basic property, the best way to measure a particular capacitor is with a technique that operates the capacitor in an environment similar to its expected use in practice. As an example, when the capacitor is to be used at RF (Radio Frequencies), and particularly at the higher ones above VHF (Very High Frequencies), e.g., at UHF (Ultra High Frequencies) or at microwave frequencies, (these days, generally 1 GHz and up), a genuine RF measurement technique (e.g., a suitable bridge) may be required, owing the unpredictable way that other reactance and losses peculiar to RF manifest themselves. For applications of capacitors at audio frequencies and at DC, measurements can often be performed with just DC techniques, although there are types of capacitor test equipment that do perform dedicated audio measurements.
These days, the degree of functionality that can be provided in what used to be called a VOM (Volt Ohm Milliammeter), or perhaps a VTVM (Vacuum Tube VoltMeter), and is finally at present a DVM (Digital VoltMeter) and its sibling the DMM (Digital Multi-Meter) means that these newer instances of electronic test equipment usually do much more than their ancestors the VOM and VTVM. In no small measure this because the best ADC (Analog to Digital Conversion) techniques are very algorithmic in nature, with the outcome that these DVMs and DMMs use a mix of processing power assisted by dedicated hardware to achieve their aims. Given that such is the case, much of the added functionality mentioned above can be provided by merely adding further algorithmic content supported with little or no extra hardware. Often the only ‘extra’ hardware is what is needed to provide a pleasant user interface, since DVMs and DMMs are not yet expected to have anything like the GUI (Graphical User Interface) that assumes the presence of the monitor, keyboard and mouse that are so familiar to today's computer users.
Furthermore, modern ADC techniques more often than not employ timing resources of modest to good capability, so that even if they are not a replacement for a good general purpose counter, many of the additional features that a DVM or DMM might have are based on an independent ability to measure the times related to when things happen, and to do things at specific times. So, the notion that the value of a capacitance can be discovered by charging it with a constant current (say, one otherwise used for the ohmmeter function) and either measuring how long it takes to reach a certain voltage, or, charging it for a certain known time and then measuring the resulting voltage, (both of which exploit the relationship I=C(dv/dt)) is a good example of the kind of auxiliary functionality that is often found in such test equipment of today. It will be noted that this class of added feature for a DC digital meter arises from the confluence of the ability to measure time, voltage, set a constant current, and perform arithmetic computation.
Just as the RF community appreciates certain capacitor properties that can only be measured under RF conditions, today's fast DVMs and DMMs can provide a variety of useful low frequency and ‘static’ parameters about the capacitances they measure. But not all measurement techniques measure all such parameters; some techniques ignore this one, while others ignore or cannot measure another. For example, the simple constant current method mentioned just above does not, without elaborate precautions, or requiring computational resources in excess of the benefit obtained by the result, account for an equivalent parallel resistance (RP) within the specimen whose capacitance CX is being measured. Not only is RP not reported, but its presence can cause the measured capacitance CX to be in error.
The reason for this is as follows. One would ordinarily expect a capacitance charged with a constant current source to exhibit a liner change in voltage (a ramp). It will, if it is an ideal capacitor and the current is truly constant. RP is one of a capacitor's potential warts that will cause the charging curve to assume an exponential form. It is not difficult to appreciate the underlying difficulty. As the voltage across the capacitor rises, ever more current flows through RP, since there is more voltage across it. But the total current flow for both the capacitor and RP is just that provided by the constant current source. That current is now divided, and ever less of the total goes to the capacitor as its voltage increases. Thus, the capacitor is NOT being charged with a constant current, after all! The decreasing amount of current entering the capacitor means that the voltage across the capacitor is rising ever more slowly. Instead of a ramp there will be an exponential curve influenced by the value of RP. In principle, one can find from measurement what the particular equation is that describes the particular observed behavior, and then solve it for CX and RP. The reality is that it is an ugly process, requiring a considerable amount of computational horsepower—much more than a mere DMM would ordinarily command. So much for the straightforward brute force approach!
As designers and purveyors of fine quality test equipment, we should like have an algorithmic technique that is based on the constant current trick for charging a capacitor to measure its capacity, but we should also like to be able to measure RP and report correct values for both CX and RP that are commensurate with the resolution that our DMM exhibits for other measurements that it can make. Such a capability should be fast, so that averages for repeated measurements can extend resolution and are readily available without a perceived penalty of undue delay, and should lend itself to easy auto-ranging over many decades. And it in keeping with that, as well as with a desire for economy, the algorithmic aspect of such a technique should not be a resource hog; it would be most desirable if it returned accurate answers with just the use of the four basic arithmetic operations of addition, subtraction, multiplication and division. That is a significant wish list. What to do?